Gas phase reaction of F − with chlorocyclopropane. Ab initio study of elimination and substitution

Abstract

The stereochemistry of elimination in chlorocyclopropane was investigated using ab initio (6-31(+)G ∗//HF/6-31(+)G ∗) calculations. The competitive S N2 route was also considered. Transition states were located for the E2(anti), E2(syn) and S N2 reactions. The three-membered ring has a pronounced effect on the syn/anti preference of E2 elimination; the barrier of syn elimination (22.2 kcal mol −1) is only 3.6 kcal mol −1 larger than that of anti elimination. The small syn/anti difference is related to the inherent periplanarity of the syn transition state and the torsional ring strain of the anti transition state. Comparing the reaction of F − with cyclopropyl, cyclopentyl and cyclohexyl chlorides, it was found that the tendency for anti elimination is especially reduced in the three-membered ring because of the non-periplanar conformation of the anti transition state. According to theory, the initial reactant complex can react further via the E2(anti) pathway or S N2 substitution. The barrier to substitution is 2.5 kcal mol −1 and the S N2 pathway dominates over elimination in this system. At the MP2/6-31(+)G ∗//HF/6-31(+)G ∗ level, the S N2 pathway has a lower activation barrier by 7.3 kcal mol −1 compared with the E2(anti) pathway.